01 - Combine beef with soy sauce, oyster sauce, rice vinegar, cornstarch, and sesame oil in a bowl. Toss well and let marinate for 10 minutes.
02 - In a small bowl, whisk together soy sauce, oyster sauce, hoisin sauce, chili garlic sauce, brown sugar, and water. Set aside.
03 - Heat 1 tablespoon vegetable oil in a large wok or skillet over high heat. Add marinated beef in a single layer and stir-fry 2 minutes until just browned. Remove beef and set aside.
04 - Add remaining 1 tablespoon oil to pan. Add onion, red and green bell peppers, garlic, and ginger. Stir-fry for 2-3 minutes until slightly tender yet crisp.
05 - Return beef to pan, pour in prepared sauce and toss to combine. Cook for 1-2 minutes until beef is cooked through and sauce thickens slightly.
06 - Garnish with sliced spring onions and serve immediately with steamed rice or noodles.